1. Field
One or more aspects of embodiments according to the present invention relate to antenna beam shaping and more particularly to methods of providing a broadened beam with large spoil factors using an actively electronically scanned array antenna.
2. Description of Related Art
Large actively electronically scanned array (AESA) antennas are commonly used in intelligence, surveillance, and reconnaissance (ISR) radar systems, where such antennas provide improved resolution angle and the ability to transmit high power, and to distinguish slow-moving targets from stationary ones. Large antennas are also capable of forming narrower transmitting and receiving beams or, equivalently, an antenna pattern with a narrow main lobe.
In synthetic aperture radar (SAR), if the signal processor has sufficient power, a wide antenna pattern is preferred, as it results in better mapping efficiency, i.e., an improved rate of mapping the ground. Moreover, it is desirable that the product of the transmitting and receiving antenna patterns, referred to herein as the TR pattern, be nearly uniform over the main beam, and that it have low sidelobes, making it possible to use a relatively low pulse-repetition frequency for a given beam width.
The antenna pattern of an array can be adjusted by setting the array element transmitting and receiving weights, a process also known as selecting a taper. The transmitting weight for an array element is the complex coefficient by which the transmitted signal is multiplied before being transmitted by that element; the receiving weight for the element is the complex coefficient by which the signal received by that element is multiplied before being combined with the corresponding signals from other array elements. The weights, being complex numbers, may be represented as a magnitude, which may be referred to as an amplitude, and a phase.
Prior art beam broadening approaches include selecting transmitting and receiving weights the phase of which varies quadratically across the array. For a planar AESA antenna, such weights have the effect of generating curved wave fronts, and a wide beam. This approach, however, is limited by ripple in the main lobe, which becomes unacceptable, for SAR applications, for beams broadened by more than 100%, i.e., by more than a factor of two. There is a need, then, for a beam broadening approach which generates broad beams with acceptable main lobe ripple.